1. Field of the Invention
This invention relates to the use of etherified polygalactomannan gums as carbonaceous slurry stabilizers. More particularly, this invention relates to the use of (1) the alkyl ether of a polygalactomamman alone, or in combination with xanthan gum or (2) the hydroxyalkyl ether of a polygalactomannan in combination with xanthan gum to stabilize a carbonaceous slurry.
2. Statement of the Related Art
U.S. Pat. No. 4,242,098 discloses the use of certain water soluble polymers, e.g. xanthan gum, hydroxypropyl guar or carboxymethyl hydroxypropyl guar gum, in an aqueous coal slurry to permit the extrusion, pumping and transport of higher solids content aqueous coal slurries.
A similar teaching is found in U.S. Pat. No. 4,375,358 which discloses the use of xanthan gum as a gelling agent in combination with certain cellulose or starch derivatives. U.S. Pat. No. 4,436,528 discloses slurries using alginates or polygalactomannan gums or their derivatives, i.e. guar and locust bean gums and derivatives such as those disclosed in U.S. Pat. No. 4,242,098 referred to earlier.
U.S. Pat. No. 4,441,889 discloses examples of stable coal-aqueous mixtures containing both xanthan gum and guar gum along with a nonionic surfactant. The examples contain two different ratios of xanthan gum to guar gum: 2 parts xanthan to 1 part guar and 10 parts guar to 1.4 parts xanthan.
U.S. Pat. No. 3,748,201 discloses the use of blends of xanthan gum and hydroxyalkyl ethers of guar gum to thicken printing pastes, explosive slurries and drilling muds. Similarly U.S. Pat. No. 3,728,173 discloses a slurry explosive containing a mixture of guar gum and xanthan gum.
WIPO publication number WO83/04047 discloses that a stable aqueous coal slurry can be prepared by the addition of a surface active agent having a hydrophilic chain composed of between about 40 to about 200 ethylene oxide units. The publication states that a slurry having a yield-pseudoplastic character is undesirable for pipeline transport. Thickeners such as xanthan gum are known to produce aqueous solutions having the yield-pseudoplastic character.
Ideally, a slurry containing carbonaceous material and a liquid should be stable under the dynamic conditions it might encounter during pumping and shipping and under the static conditions of bulk storage. In the transportation of such a slurry, (such as by barge, marine tanker, rail car, or truck tank car) various stresses might be encountered. The vibrations to which the slurry will be subjected will vary in both amplitude and frequency. Thus, for example, during shipboard travel where there is relatively little clearance between the hull and the harbor or river bottom, severe shaking of the ship will be caused by pressure waves from the ship's propellers. When shipboard travel occurs on the open water, by comparison, there will be more roll, and some pitch, but the amount of high frequency vibration should be relatively small. The amount of motion a slurry is subjected to during any type of travel should vary with the extent of confinement of the slurry: full tanks would slosh very little, and partially empty tanks would slosh considerably more.
One problem encountered with many prior art coal-water slurries is that they become unmixed when they are subjected to vibration, with disastrous consequences. When such a slurry is being pumped, the solid particles settle out of the slurry to the bottom of the pipe, thereby changing the solids content and other properties of the slurry; often a hard pack of solid material is formed which is difficult or impossible to readily resuspend in the slurry.
It is probable that, to some extent, some settling will occur in coal-liquid slurries subjected to vibratory conditions. It is desirable to minimize the degree of settling. It is also desirable to provide a slurry wherein, even if settling does occur, the settled particles form a "soft pack" (as opposed to a hard pack) and can be easily resuspended in the slurry.
The coal-water mixtures are employed as fuels in that they may be atomized into small droplets and high surface area per volume ratios which provide for efficient conversion and stable ignition.